fix: login loading overlay, deduplicate getCookieDomain (F-18, F-11)

- Login page shows LoadingOverlay during redirect instead of blank screen
- Extract getCookieDomain() to shared lib/utils/cookies.ts

lib/utils/cookies.ts

@@ -0,0 +1,9 @@
+// * Determine cookie domain dynamically.
+// * In production (on ciphera.net), we share cookies across subdomains.
+// * In local dev (localhost), we don't set a domain.
+export const getCookieDomain = (): string | undefined => {
+  if (process.env.NODE_ENV === 'production') {
+    return '.ciphera.net'
+  }
+  return undefined
+}